1. Field of the Invention
The present invention relates to, for example, a fault locating method for a submarine optical communication system, having a plurality of submarine equipments, such as repeaters or branching units, connected in series on the cable, and being supplied an operating power through the feed line provided in the cable, and relates to submarine equipments adaptable for the fault locating method.
2. Description of the Related Art
In a submarine cable communication system for effecting communication among multi-points by using submarine cables such as submarine optical fiber cable, a plurality of submarine equipments is connected sequentially in the submarine cable at predetermined intervals.
Submarine equipments, such as repeaters for repeating light wave signals or branching units for branching optical fibers, are installed in the sea water, and fed electrical power to operate main processing circuits thereof for transmitting light wave signals, through a feed line conductor contained in the submarine cable by constant-current power-supply units placed at cable landing stations.
There are two types of the constant-current serial feeding system, one type is a bidirectional type in which the direction of the feed current inverses if necessary, and is useful in the multi-point communication system in which the fault section of the cable should be disconnected from normally operating cable route. The other type is a unidirectional type in which the feed current flows only in one predetermined direction.
At least one power unit is provided in each submarine equipment to take out operating power from the feed line, and the power unit has a structure adaptable for the current feeding type.
Meanwhile, when the current supply to the submarine equipment is stopped due to an open-circuit or a short-circuit of the feed line, it is necessary to detect quickly, from the landing station, the location of cable fault occurred, in order to repair the broken cable for fast recovery of the communication system.
There have been conventionally used three types of fault locating methods in order to detect the location of the fault in the feed line; a resistance-measuring method for short-circuit fault, a pulse-echo method and capacitance-measuring method for open-circuit fault.
In the resistance-measuring method, the resistance of the feed line conductor of living section of the cable is measured from the landing station via the loop path consisting of the feed line and the sea-ground returning path, then the distance to the fault location is obtained by using the measured resistance value and previously known resistivity per unit length of the feed line conductor.
In the pulse-echo method, the location of the fault can be obtained by the measurement of the going-and-returning transmission time of low frequency pulse through the feed line between the measuring equipment at the landing station and the fault point.
In the capacitance measuring method, the electrostatic capacitance of an equivalent capacitor formed in the submarine cable is precisely measured, regarding the submarine cable between the landing-station and the open point as a co-axial condenser having a center conductor of the feed line surrounded dielectric materials and an outer conductor of the sea water ground.
In order to apply the capacitance measuring method for an open-circuit fault locating on the submarine cable communication system adopting above mentioned serial constant current feeding system, as the feed line of submarine cable is divided into many sections by power units of submarine equipments, the total capacitance of equivalent capacitors each of which is formed on the individual cable section, must be precisely measured.
The existence of power circuits connected in series in the feed line causes a deterioration of the accuracy of capacitance measurement of the submarine cable.